On the Importance of Dispersed vs Solubilized Oil - ACS Publications

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Stability of Aqueous Foams in the Presence of Oil: On the Importance of Dispersed vs Solubilized Oil Jongju Lee, Alex Nikolov, and Darsh Wasan* Department of Chemical and Biological Engineering, Illinois Institute of Technology, Chicago, Illinois 60616, United States S Supporting Information *

ABSTRACT: Aqueous foams containing oils are encountered in a variety of processing operations. The present study investigates the influence of both the dispersed and solubilized oils, and the surfactant concentration (above the critical micelle concentration) on the stability of an aqueous foaming system. The oil spreading, entering, and bridging coefficients were calculated based on the measurements of the surface and interfacial tensions. The stability of a foam cell, the coalescence time of an oil drop at the air−liquid interface, and the second virial coefficient of solutions containing solubilized oil were monitored to explain the stability of the aqueous foam. Results showed that the spreading, entering, and bridging coefficients are unable to explain the effects of the type of oil (aliphatic or aromatic) added or the effects of the surfactant concentration on the rate of destabilization of foams. However, good correlations were found between the foaming ability (or inability) of the two types of oil, the surfactant concentration used, the stability of the foam cells containing oil drops, and the change in second virial coefficients.

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INTRODUCTION This paper is dedicated to Professor L. T. Fan, an outstanding engineering educator, mentor, and inventor. One of the authors (D.W.) has known him for over 30 years and admires his fundamental and applied contributions to chemical engineering. Two-phase foams are dispersions of gas bubbles in liquids containing surfactants, proteins, or macromolecules. The bubbles are separated by liquid films or lamellae. Foams containing two phases are common. However, three-phase foams containing dispersed oils are also encountered in several applications: food foams, foam mobility control processes in enhanced oil recovery, and antifoamers. The stability of three-phase foams has been discussed by a number of authors,1−10 most recently in reviews by Miller11 and Denkov.12 Ross and McBain13 proposed that the spreading of the oil phase at an oil−air interface is an important factor in the stability of the foam. Robinson and Wood14 defined a spreading coefficient (S) and an entering coefficient (E) as follows: S = σw/a − σw/o − σo/a

for this inconsistency is that the classical theories do not take into account the stability of the aqueous film that exist between the air−water surface and the surface of an oil drop approaching it. Nikolov et al.16−18 were the first to observe the presence of this “pseudoemulsion” film (i.e., an asymmetrical film between an oil drop and the water−air interface) in their studies with enhanced oil recovery foams. According to these authors, it is the stability of this asymmetric pseudoemulsion film that controls the stability of the foam containing dispersed oil. Here, we present the results of our experimental study on the effects of the oil type (polar verses nonpolar) on foam stability at two different micellar concentrations using an anionic surfactant. The stability of the asymmetric pseudoemulsion film and the stability of a foam cell as well as the second virial coefficients were examined in order to explain the effects of both types of oil and the surfactant concentration on an aqueous foaming system.

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EXPERIMENTAL SECTION Materials and Preparation of Solutions. We used n-dodecane (the density at 20 °C is 0.75 g/cm3) and toluene (the density at 20 °C is 0.867 g/cm3). The toluene was obtained from Acros, and the n-dodecane was from Aldrich Chemical Co. An anionic surfactant, sodium dodecyl sulfate (SDS), was supplied by Fisher; the surfactant molecule has an average molecular weight of 288.38. We used 0.03 and 0.06 M surfactant solutions. These concentrations are higher than the critical micelle concentration (Cmi = 1 × 10−2 M). The surfactant solutions were prepared by dissolving SDS in

(1)

and E = σw/a + σw/o − σo/a

(2)

where σw/a is the surface tension of an aqueous solution, σo/a is the surface tension of the oil phase, and σw/o is the interfacial tension between the aqueous solution and the oil phase. Garrett15 introduced the concept of bridging coefficient as a criterion for the stability of the foam lamella. According to him, the bridging coefficient is B = σw/a 2 + σw/o 2 − σo/a 2

(3) Special Issue: L. T. Fan Festschrift

All the classical theories that use oil spreading, entering, and bridging mechanisms suggest that the stability of foams decreases with increasing spreading, entering, and bridging coefficients. However, many contradictions between the experimental data and the classical models have been reported. The prime reason © 2012 American Chemical Society

Received: Revised: Accepted: Published: 66

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deionized water using a “Milli-Q” water system deionizer from Millipore Corporation. The surfactant system was contacted with the oils and pre-equilibrated for more than 10 days. During pre-equilibration, the oil phase was dispersed into the aqueous surfactant solution as drops and left overnight for a phase separation in a glass container. We used suction repeatedly to remove the pre-equilibrated oil from the top of the container. Surface and Interfacial Tension Measurements. A Kruss tensiometer using a platinum Wilhelmy plate was used to measure the surface tension, and the drop shape analysis method was used to measure the interfacial tension. These measurements were used to determine the spreading, entering, and bridging coefficients. Foam Stability Measurements. The foams were produced from micellar surfactant solutions of sodium dodecyl sulfate using the Bartsch method. The three-phase foams (with two oils) were generated in a graduated glass cylinder (250 mL) by shaking 50 mL of the surfactant solutions with 0.2 vol% of the oil phase 20 times. The foam height was monitored as a function of time. The stability of foams formed by shaking aqueous solutions containing SDS swollen micelles (micelles solubilizing oil) was also determined. Oil DropletFoam Cell Interaction. We used a transmitted light microscope with a double plane parallel glass cell and a fixed gap to visualize the process of the oil droplet and foam cell interaction (Figure 1). A foam sample taken by a

Figure 2. Sketch of the experimental setup to study the stability of asymmetric pseudoemulsion film.

Figure 1. Sketch of the experimental setup to study the stability of foam lamella in the presence of oil.

Figure 3. Sketch of the experimental setup to study the spreading of emulsified oil droplets.

dispersal pipet was placed between two glass slides with a gap of 200 μm and the two-dimensional foam was observed through the miccope. This method of visualization is similar to the one previously described by us.18,19 A halogen light source was used to create a beam of light that passed through a monochromatic filter. The filtered beam passed

through a plane parallel glass cell containing a two-dimensional foam structure, allowing for the visualization of three-phase foam structure. A digital video camera was used to record the foam cell interactions with the oil droplets. Stability of the Asymmetric Pseudoemulsion Films. An aqueous film was formed between the air−solution surface and 67

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an approaching oil drop. The stability of this asymmetric film was assessed by monitoring the time it took for the drop to coalesce at the interface (i.e., until the pseudoemulsion film broke). The experimental setup is shown in Figure 2. An oil drop was injected into the glass cylinder (1.5 cm in diameter and 3.5 cm in height) and covered to prevent evaporation; the time of the drop coalescence at the air−solution surface was then recorded. Photographs in Figure 2 depict the top and side views of the coalescing drop. Spreading of Oil Drops at the Solution−Air Surface. The experimental setup is shown in Figure 3. Oil drops were preequilibrated with the surfactant solution and were placed below the solution−air interface. After the pseudoemulsion film ruptured, the oil lenses formed at the air−solution surface were visualized using both transmitted and reflected light microscopy. Videos of the observations were recorded. Turbidity Measurements. Turbidity measurements were made using the Hach 2100A turbidimeter. The instrument was calibrated using an aqueous solution of a nonionic surfactant, NEODOL 25-12, obtained from Shell Chemicals. The calibration is shown as a Debye plot in Figure 4.

Refractive index measurements were made using a Fisher Refractometer at a temperature of 26 °C ± 1 °C.

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RESULTS AND DISCUSSION Foam Texture and Stability. The three-dimensional foam cell texture was observed over an 8 h period, and a significant difference in the foam textures was found. The foam texture of the aqueous foaming sample formed from a 0.06 M SDS surfactant solution containing dispersed droplets of toluene was more porous than the foam texture of the sample containing dispersed n-dodecane. In contrast, we observed a dense foam created through the use of n-dodecane. This foam’s lamellae became thicker with the passage of time. Figures 5a and 5 depict the foam texture for the toluene and n-dodecane foam samples, respectively. The foam stability was measured by the decrease in the foam height with time. The decrease in the foam height with time was more pronounced in the sample containing dispersed n-dodecane than that containing toluene. About 80% of the foam in the presence of n-dodecane collapsed in about 1 h, whereas the foam height in the presence of toluene decreased slowly in a stepwise manner. The results of the foam stability tests are shown in Figure 6 for both the oil-free surfactant solutions and samples

Figure 4. Calibration curve for turbidity measurements. (c−c0) is the concentration of surfactant present as micelles, (τ−τ0) is the excess turbidity of the solution over the solvent, and H is related to the refractive index of the solvent.

Figure 6. Foam stability (foam height vs time) measurements of SDS surfactant solutions with two types of dispersed oil at two different concentrations of surfactant.

Figure 5. Texture of 0.06 M SDS foam in the presence of (a) dispersed toluene and (b) dispersed n-dodecane. 68

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Foam Cell Stability. The basic unit of any foam system is the foam cell. This foam cell is built of foam lamellae, as depicted in the photograph in Figure 7. The interaction of the dispersed oil droplets with the foam lamellae was studied by viewing a foam sample between two glass slides under a microscope (Figure 1). Figure 8 shows photographs of the foam lamella of the foam made from the 0.03 M SDS solution in the presence of dispersed oil. The oil drops have drained from the foam cell and accumulated within the Gibbs-Plateau borders (PB). The size of the toluene drops vary from 10 to 60 μm in diameter (Figure 8a), whereas the size of the n-dodecane drops are 60−100 μm in diameter. Once trapped in the restricted geometry of the PB, the shrinking Plateau borders squeeze the oil drops, and further increasing the radius of the pseudoemulsion film results in a rupturing of the film formed between the solution−air interface and the oil droplet (i.e., the asymmetrical pseudoemulsion film) in the borders. This causes the destruction of the whole frame of the foam cell in the case of the n-dodecane dispersed sample. But in the sample containing dispersed toluene, smaller droplets coalesce into larger droplets and the foam cell structure is more stable. Therefore, the interaction between oil droplets in the Plateau borders and liquid film plays an important role in determining foam stability. The dynamics of this phenomenon are depicted in the movie available in the Supporting Information available accompanying this paper. Oil Droplet−Film Interaction. The stability of the asymmetric pseudoemulsion film was assessed by monitoring the time it took for a single drop of oil to coalesce at the solution−air interface using the experimental arrangement shown in Figure 2. We found that the coalescence time of n-dodecane drop was about 12 times less than that for the toluene system, indicating that the film formed between the solution−air surface and the approaching n-dodecane drop would be much less stable than that for the toluene drop. This would suggest that the foam lamella formed from a surfactant solution containing dispersed n-dodecane would break faster than the lamella containing dispersed toluene. Consequently, the foam would be less stable for the n-dodecane system. This is exactly what we observed, and the data for the foaming systems made from 0.03 M SDS solution supporting these observations are shown in Table 1. Table 2 shows the surface and interfacial tensions as well as the entering (E), spreading (S), and bridging coefficients (B) for all

Figure 7. Trapped oil droplets inside of Plateau borders.

containing dispersed oils. The order of the stability of these foams is as follows: oil-free samples samples with dispersed toluene samples with dispersed n-dodecane. In the case of the dispersed toluene system, the stability of the sample formed from 0.06 M SDS is greater than that formed from 0.03 M SDS. In contrast, the sample containing dispersed n-dodecane shows a slight difference in stability. In a previous study, Nikolov et al.20,21 found that a single foam film formed from the SDS micellar solution exhibited a layering structure that slows down the film drainage, resulting in an increase in the foam stability with an increase in the micellar concentration. According to them, the aqueous foam film formed from the 0.03 M SDS solution contained 3 layers whereas the film formed from the 0.06 M solution contained 5 layers. Therefore, the foam made from a higher micellar concentration is more stable than that made from a lower micellar concentration. This is consistent with the results we report in Figure 6. We also conducted the foam stability tests with the preequilibrated aqueous solutions containing swollen micelles. We observed that the stability of the foam containing solubilized toluene differed very little from the toluene-free case, indicating the importance of the dispersed oil droplets in controlling foam stability. However, the foam containing solubilized n-dodecane was found to be as unstable as that containing drops of n-dodecane. This suggests that the solubilized oil and not the dispersed oil plays a much more important role in controlling the stability of the aqueous foaming system containing n-dodecane.

Figure 8. Oil droplets inside the Plateau borders (2D foam cell): (a) toluene drop, (b) n-dodecane drop. 69

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is a measure of the kinetic stability of the pseudoemulsion film, is more than 10 times less for n-dodecane than that for toluene. This indicates that the spreading coefficient, which is a thermodynamic quantity, is not sufficient to explain the dynamic process of the foam lamella stability. Consequently, the spreading coefficient is unable to explain the foam stability. Second Virial Coefficient. The results from the turbidity and reflective index measurements were represented as Debye plots.22 The value of H(c − c0)/(τ − τ0) was plotted against the surfactant concentration. The term (c − c0) indicates the concentration of surfactant present as micelles. The term (τ − τ0) is the excess turbidity of the solution over the solvent. We use the slopes of the Debye plot to calculate the values of the second virial coefficient. The values of the second virial coefficient are listed in Table 3 along with the order of the foam stabilities.

Table 1. Effect of Dispersed Oil Droplet on Coalescence Time, Foam Lamella Breaking Time, and Foam Stability system single oil droplet coalescence time (s) average time of foam lamella breaking (min) foam stability: 80% of foam breaking time (h)

0.03 M SDS in the presence of n-dodecane

0.03 M SDS in the presence of toluene

5.9 ± 1.2

76.4 ± 3.3

1.5

over 15

1

7.8

Table 2. Surface (σw/a, σo/a) and Interfacial (σw/o) Tensions, Spreading (S), Entering (E), and Bridging (B) Coefficients in the Studied Systems system concentration (mol/L) σw/a (mN/m) σo/a (mN/m) σw/o (mN/m) E (mN/m) S (mN/m) B (mN2/m2)

SDS in the presence of n-dodecane droplet 0.03 38.3 25.3 0.07 13.1 12.9 826.8

0.06 37.3 25.3 0.07 12.1 11.9 751.2

SDS in the presence of toluene droplet 0.03 38.3 28.4 0.03 9.9 9.8 660.3

0.06 37.3 28.4 0.03 8.9 8.8 584.7

Table 3. Values of Second Virial Coefficient and the Order of Foam Stability system SDS (oil free) SDS + toluene SDS + n-dodecane

second virial coefficient −2

1.31 × 10 1.46 × 10−2 −7.58 × 10−4

foam stability stable stable least stable

The foam stability in the presence of oil not only depends on the dispersed/emulsified oil but also on the solubilized oil. Our previous study has shown that the foam stability drops after the foaming solution is equilibrated with the oil.8 This is due to the presence of oil solubilized within the surfactant micelles. Lobo et al.9 has shown that when oil is solubilized within the micelles, the van der Waals attractive forces are increased between the micelles, resulting in a reduction in the value of the second virial coefficient. The foam stability is found to decrease with a decrease in the value of the second virial coefficient. In other words, a positive second virial coefficient represents an overall

the systems studied. These coefficients are all positive which, according to the classical theory, indicates that the entering and spreading of the oil drop are thermodynamically favorable. Our microscope observations, using the experimental setup shown in Figure 3, showed that the oil drop did spread at the solution−air interface as lenses (see Figure 9), but the degree of spreading for the dispersed drop of n-dodecane was larger than that for the toluene drop. It should be noted that the difference between the spreading coefficient for the n-dodecane drop and that of the toluene drop is only about 20%, but the coalescence time, which

Figure 9. (a) Formation of n-dodecane lens after breaking of pseudoemulsion film (SDS concentration of 0.03 M). (b) Formation of toluene lens after breaking of pseudoemulsion film (SDS concentration of 0.03 M). 70

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was more stable than that containing n-dodecane. A good correlation was obtained between the stability of the foam cell containing oil drops of toluene and n-dodecane and the relative foam stability. We also measured the foam stability in the presence of the solubilized oil. Our results show that in the case of toluene, the swollen micelles do not affect the foam stability, but the oil drops do alter the foam stability. In contrast, for the n-dodecane case, the oil drops do not alter foam stability, but solubilized oil greatly influences foam stability. We have also measured the second virial coefficients and found that the second virial coefficient is positive for the micelles containing toluene, indicating repulsive interaction between the micelles; whereas the second virial coefficient is negative for the micelles containing n-dodecane, thereby signifying attractive interactions. Consequently, the foam is much less stable in the presence of n-dodecane. In summary, the classical spreading theory can neither explain the foam stability in the presence of solubilized oil only, nor the rate of destabilization of foams in the presence of oil droplets. It is concluded that in foam stability, the relative importance of the dispersed oil vs the oil solubilized in the micelles depends on the stability of the pseudoemulsion film and the second virial coefficient.

repulsive interaction between the micelles. On the other hand, a negative second virial coefficient represents an overall attractive interaction between the micelles. On the basis of the values of second virial coefficients in our studies, we are able to elucidate the reason for different foam stabilities in solutions containing only swollen micelles. In the case of the SDS solution pre-equilibrated with n-dodecane, the second virial coefficient is negative (attractive intermicellar forces), while the system pre-equilibrated with toluene has a positive second virial coefficient (repulsive intermicellar forces). According to our results, the effect of swollen micelles is dominant in the case of the SDS solution pre-equilibrated with n-dodecane. Even when we removed the oil droplets from the solution, the foam formed with solubilized oil which contained only swollen micelles, showed low stability like the foam in the presence of dispersed n-dodecane. On the other hand, the effect of the oil drop is dominant in the case of the SDS solution preequilibrated with toluene. When we took out pre-equilibrated toluene droplets from the pre-equilibrated solution, the foam stability of the solution saturated with toluene increased. The second virial coefficients of the solution pre-equilibrated with toluene and the pure SDS solution have similar positive values. We observed that the toluene droplet played a significant role in controlling foam stability, rather than the swollen micelles. Hence, the stability of the pseudoemulsion film and the size of the film (i.e., drop size) are important. Classical spreading theory is not able to explain the foam stability of the pre-equilibrated solution without dispersed oil droplets. The spreading coefficient of SDS in the presence of the n-dodecane droplet has a positive value. According to the classical model, spreading oil drops make foam unstable. When we separated n-dodecane droplets from the pre-equilibrated solution, the foam stability of this solution was similar to that of the foam in the presence of n-dodecane. In this system, there were no dispersed n-dodecane droplets (i.e., no spreading), but the system was still unstable because of the swollen micelles. In this respect, the spreading coefficient does not explain the foam instability of the pre-equilibrated solution without the dispersed oil.

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ASSOCIATED CONTENT

S Supporting Information *

Movie clip: relative stability of a foam cell in the presence of toluene and n-dodecane. The movie clip depicts the interaction of the dispersed oil with a two-dimensional (2D) foam cell. The movie frame seen on the left depicts the dynamics of the drops of toluene interacting within the 2D frame cell. Dispersed toluene droplets tend to move out of the foam lamellae and accumulate in the Gibbs-Plateau borders. The droplets coalesce into larger droplets over time, and the foam cell structure does not collapse in the presence of droplets. Very different dynamics are observed in the case of the dispersed n-dodecane, seen on the right side of the screen. The accumulated droplets in the Plateau borders are larger than the toluene droplets. Furthermore, the droplets cause the destruction of the whole frame of the foam cell. Consequently, the foam cell containing droplets of n-dodecane is relatively less stable than that of the toluene system.

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CONCLUSIONS Foam stability in the presence of dispersed oil was studied by using the Bartsch test method. We examined the effects of two types of oils, aromatic (toluene) and aliphatic (n-dodecane), on the stability of aqueous foams made from solutions of sodium dodecyl sulfate, 0.03 M (i.e., 3.6 times CMC) and 0.06 M (i.e., 7.3 times CMC), respectively. It was observed that dispersed ndodecane decreased foam stability much more than toluene. Results confirmed the past studies that the classical spreading− entering theories do not explain the rate of destabilization (or stabilization) of aqueous foams containing dispersed oils. In order to reveal the effect of the oil droplet on foam stability, we measured the single oil drop coalescence time at the solution−air interface. The coalescence times for the drops of toluene and n-dodecane correlated well with the foam stability. We also conducted microscopic observations of the interactions of the oil drops within the foam lamellae and the Gibbs-Plateau borders. It was observed that the oil droplets moved to the Plateau borders of the draining foam and got trapped in the thinning borders. Over time, the drops coalesced to form larger droplets. The aqueous asymmetric (i.e., pseudoemulsion) film formed between the trapped oil and the air−solution interface ruptured, triggering the foam cell’s collapse. The foam cell containing drops of toluene

This information is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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